1. Field of the Invention
This invention relates to the field of high density animal housing systems, for example as used to house laboratory animals such as experimental mice, wherein each cage is individually ventilated and the occupants of each cage are to be isolated from airborne particles from other cages which may carry contagion.
2. Prior Art
High density animal caging systems are known, for example as disclosed in U.S. Pat. No. 4,343,261 - Thomas, wherein the animals are housed in box-like enclosures which are airtight on their sides and bottoms and are adapted to fit to ducted shelves such that open tops of the cages fit against the ducted shelves in the area of air supply and air exhaust openings. When the cages are fitted into place under the shelves, for example being slid under the shelves along complementary supporting flanges on the cages and shelves, respectively, air flow is established through the cage. The internal ducts of the shelves are connected to a powered airflow means typically including a HEPA filter or the like for removing particulate matter. Clean air is carried along the shelf ducts to each cage and used air, which may also contain particulate matter, is carried away via the exhaust ducts. This arrangement accomplishes individual ventilation of the cages, allowing for high density animal housing as appropriate for laboratories and the like.
Animal cages require regular maintenance. Used bedding material must be removed, and food and water must be added. If the animals are housed in open topped cages, there is a period of exposure to particulate material from other animals when a cage is removed from the ducted shelf rack for such maintenance. Particulate matter in the form of airborne dust and dander can carry various diseases, a very serious example being infantile diarrhea in mice, which can cause extensive mortality in breeding colonies. Typically, clean cage boxes and bedding are made available and upon removal of a cage from the ducted rack the animals are transferred to a clean cage box with required food and water, whereupon the cage is replaced in the rack. The original bedding is discarded and the original cage is cleaned. The transfer operation can be conducted in a protected environment, for example in a hooded maintenance enclosure with a laminar air flow pattern to reduce the escape of dander, bedding particles and other potential disease vectors. However, the cage and its occupants still are exposed at least when being slid into or out of engagement with the ducted rack. According to the aforesaid patent to Thomas, it is possible to include a cover on the cages to prevent this period of exposure. A cage cover having valves which align with the air inlets and outlets is placed over each cage, the cover fitting between the cage and the shelf. The valves are spring biased to close and are opened against the spring bias by contact with the underside of the shelf. When the cage is inserted, the valves open; and when the cage is removed, the valves close.
The particular valves employed in Thomas have cylindrical valve bodies movable in tubular valve housings in a direction perpendicular to the plane of the cage cover for opening and closing the air flow path. The remainder of the cover is airtight plastic, being sealed to the perimeter of the cage by an appropriate resilient seal means which engages with the cage along its upper edges. Helical springs urge the valve bodies upwardly into the closed position. In the open position, a piece of filter material within the valve prevents any particulate material from being drawn out of the cage and into the exhaust ducts of the rack.
The valves in the Thomas disclosure are effective, but expensive to provide and to maintain. Moreover, when the valves are closed, there is no possibility of air flow to the animals, presenting a danger of suffocation if the cage is left too long out of the ducted rack and/or if a valve is not operated properly, for example due to clogging or due to an improper fit between the cage and the rack. In order to ensure a proper fit, the cage flanges and the complementary shelf flanges must be relatively precisely positioned and dimensioned. The animal maintenance technician must ensure that each cover is properly in place such that the covered cage will fit into the shelf. Each cage must be inserted fully to align the valves and their respective ducts. Care must be taken that the valves do not become clogged, because air flow can occur only through the valves.
Animal housing systems are known wherein the cages are covered by filter material rather than by an airtight cover. Examples are disclosed in U.S. Pat. Nos. 3,343,520 - Schwarz, Jr.; U.S. Pat. No. 3,528,277 - Lee et al; 4,480, 587 - Sedlacek; and British Patent 2,065,440 - Bernardini. The covered cages are not adapted for use in cage racks with internal air ducts. In filter covered cages the body heat of the cage occupants produces thermal currents that cause an exchange of air with the outside. Over time, filter covers can become clogged with particles, reducing the air exchange; however, the surface area available for air exchange is much larger than the surface area available through a valve as in Thomas.
U.S. Pat. No. 4,593,650 - Lattuada discloses a filter covered cage with a powered air inlet which is plugged into the cage, thereby forcing air outwardly through the cage and ensuring ventilation. However, the plugged connection requires individual attention in aligning the cage and the air supply plug, and in ensuring an operative connection.
Filter covers which are self supporting are typically made of bonded fibers and adhesive. The adhesive tends to occupy spaces between the fibers and to block air flow. The foregoing patents to Sedlacek, Lattuada and Bernardini support or sandwich a sheet of filter material between frame elements, for example perforated plates. The perforated plates also tend to limit air flow. Moreover, such composite structures are unduly thick. The filter covers also must be assembled and used filter sheets must be replaced, making it difficult to simply process a used filter cover via a cleaning process such as autoclaving.
The present invention employs a cage cover wherein a filter material of spunbonded polyester, which is quite durable, is bonded directly to the upper surface of a limited frame. The frame has peripheral edges that snap over the top edge of the cage to ensure a good seal. The vertical thickness of the frame is minimal, allowing the cover to be placed over a cage fittable in a cage rack. The area of the cover over the open top of the cage is substantially defined by filter material, which extends over the full area of the top of the cage and frame, save an opening for a valve to align with the air supply opening in the rack. Thin webs extend across the cage top from the periphery of the frame of the cover to support the filter material without substantially blocking the air exchange area. A valve structure in the form of a spring leaf is carried in the frame by means of support webs that extend inwardly from the frame edge, the spring leaf blocking and unblocking the opening in the filter material for air supply when the cage is removed and inserted into the cage rack, respectively. Air is exhausted to the rack exhaust opening directly through an area of the filter material without webs. An upward protrusion which may be attached to the spring leaf or simply placed on the spring leaf, contacts the underside of the rack shelf for uncovering the supply opening. The filter cover as a whole is strong and effective, can be simply autoclaved for cleaning, and protects the animals from contagion while freeing the animal maintenance technician from demanding requirements of cage-cover assembly, cage positioning and the like.
The supply valve allows unrestricted air flow into the cage, eliminating the diffusion of air across the filter media as well as the associated pressure drop that would occur through the filter media if the filter material were disposed across the area of the supply duct opening. This allows the micro-environment within the cage in the rack to remain positively pressurized relative to the room in which the rack is located, even when the exhaust orifice static pressure on the filter media is raised to a level greater than the supply orifice pressure, as long as the differential pressure is less than the pressure drop across the filter media.